Do not copy expectations file to itself.

slave/skia_slave_scripts/prerender.py

Index: slave/skia_slave_scripts/prerender.py
diff --git a/slave/skia_slave_scripts/prerender.py b/slave/skia_slave_scripts/prerender.py
index bc36c07a95a25603516ccfaed7382fe4941cc71b..aa5de007b6af5cd9b8750dc7385319666922bdbe 100644
--- a/slave/skia_slave_scripts/prerender.py
+++ b/slave/skia_slave_scripts/prerender.py
@@ -133,8 +133,16 @@ class PreRender(BuildStep):
     if os.path.exists(skimage_host_expectations):
       skimage_device_expectations = self._flavor_utils.DevicePathJoin(
           self._device_dirs.SKImageExpectedDir(), skimage_expected_filename)
-      self._flavor_utils.PushFileToDevice(skimage_host_expectations,
-          skimage_device_expectations)
+      # For builders without an attached device, PushFileToDevice will fail
+      # when attempting to copy a file to itself. In this case, there is no
+      # need to copy. Only do the push when there is an attached device,
+      # which corresponds to the case that the filepaths are equal.
+      # TODO(scroggo): Once
+      # https://code.google.com/p/skia/issues/detail?id=1571 is fixed, this
+      # check can go away.
+      if skimage_device_expectations != skimage_host_expectations:
+        self._flavor_utils.PushFileToDevice(skimage_host_expectations,
+            skimage_device_expectations)
 
     self._flavor_utils.CopyDirectoryContentsToDevice(
         self._skimage_in_dir, self._device_dirs.SKImageInDir())